Device for detecting tachycardiac rhythm disturbances

ABSTRACT

A device for detecting tachycardiac rhythm disturbances, has a measuring unit for picking up and reproducing a measurement signal dependent on intracardial impedance at its output and an evaluation unit connected at the input to the measuring unit. The evaluation unit is adapted to output both a first signal which corresponds to the difference of a maximum and a minimum measurement signal within at least one predeterminable period of time and also a second signal which is dependent on the integral of the measurement signal over at least one predeterminable period of time.

The invention concerns a device for detecting tachycardiac rhythmdisturbances, comprising a measuring unit for picking up and reproducinga measurement signal dependent on intracardial impedance at its outputand an evaluation unit connected at the input to the measuring unit.

BACKGROUND OF THE ART

The invention further concerns an implantable electrostimulation devicefor the treatment of tachycardiac rhythm disturbances, comprising adetection unit, a control unit and a therapy unit which is adapted toproduce a cardioversion or defibrillation electrotherapy to betransmitted to the heart, wherein the detection unit has a measuringunit for picking up and reproducing a measurement signal correspondingto intracardial impedance at its output and an evaluation unit connectedat the input to the measuring unit and the control unit receives outputsignals from the evaluation unit and controls the activity of themeasuring unit, the evaluation unit and the therapy unit.

By virtue of the lower specific resistance of blood in comparison withthe myocardium tissue intracardial impedance varies with the volume ofblood in the chambers of the heart, which changes in the course of thecardiac cycle. Thus, the pump activity of the heart can be monitored byevaluation of an intracardial impedance measurement and the existence ofcardiac rhythm disturbances such as tachycardia or fibrillation can beinferred from variations in the amplitude pattern or the frequency ofthe periodic impedance signal.

Such a monitoring device is known from European patent application No. 0009 255 A1, to Geddes, published 2 Apr. 1980. That publication disclosesan implantable defibrillator having two measuring electrodes arranged atan axial spacing from each other at the distal end of a catheter whichis introduced into the right ventricle. A control logic unit starts anintracardial impedance measurement procedure when automatic evaluationof an ECG signal indicates the possible existence of fibrillation.Impedance is determined by means of voltage measurement between theelectrodes with an alternating current flow with a constant modulationamplitude and subsequent demodulation of the voltage signal. If theamplitude of the impedance signal indicates excessively low pumpactivity on the part of the heart defibrillation therapy is initiated.

A disadvantage with that device is that the signals from the measuringelectrodes react sensitively to interference effects by virtue of theirarrangement in a ventricle. Thus, body movement can already give rise tocontact between the electrodes and the myocardium tissue. That meanshowever that the measurement voltage and thus impedance measurement arefalsified. Unnecessary defibrillation shocks which are painful to thepatient can be the consequence of defective impedance measurement.

It is known from U.S. Pat. No. 5,427,112, to Noren, issued 27 Jun. 1995,for increasing the reliability of detection of cardiac rhythmdisturbances, to record two signals and to monitor the in-phase periodicrecurrence thereof, which is coupled to the heartbeat. In addition tomeasurement of the intracardial impedance signal, that known device alsoprovides for determining the derivative thereof in respect of time. Theimpedance signal is recorded in a parameter representation as a functionof its derivative in respect of time. The curve recorded in that way iscompared to stored pattern curves, whereupon a decision is made aboutthe necessity for and possibly the nature of a therapy. That proceduresuffers from the disadvantage that it is highly costly in terms ofcomputation and memory; it is firstly necessary to determine thederivative in respect of time of the measurement signal. The measurementvalue together with the derivative in respect of time have to be storedover at least the duration of a cardiac period. Then the phase positionof the stored data has to be determined, in comparison with a previouslystored pattern curve, and that requires extensive computations. Finallyit is then necessary to form the difference of the measured pairs ofvalues and corresponding pairs of values of the pattern curve, andultimately evaluate same on the basis of a mathematical criterion.

U.S. Pat. No. 5,179,946, to Weiss, issued 19 Jan. 1993, discloses animplantable defibrillator in which, to increase the level of reliabilityof detection of cardiac rhythm disturbances, the intracardial impedancebetween two defibrillation electrodes fixed to the outside of the heartis measured. Adequate pump activity on the part of the heart ismonitored on the basis of the impedance signal, by means of an amplitudediscriminator. As an alternative to amplitude discrimination that knowndefibrillator provides for integration of the measured impedance signal.That device is complicated and expensive in circuitry terms becauseimpedance measurement and tachycardia therapy are effected by way of thesame electrodes. Therefore, avoiding destruction of the measuring unitby the high electrical voltages which are produced in a cardioversion ordefibrillation procedure requires a protective circuit which is to beconnected between the defibrillation electrodes and the measuring unitprior to application of the therapy. A disadvantage with that knowndefibrillator is also major operative involvement which stresses thepatient when implanting the epicardial electrodes.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a reliable but lesscomplicated and expensive device for detecting tachycardiac rhythmdisturbances and a less complicated and expensive implantableelectrostimulation device for the treatment of tachycardiac rhythmdisturbances.

In regard to an device for detecting tachycardiac rhythm disturbancesthat object is attained by the features of claim 1 and in regard to animplantable electrostimulation device for treating tachycardiac rhythmdisturbances that object is attained by an device having the features ofclaim 20.

The basic idea of the invention is to increase the reliability ofdetection of cardiac rhythm disturbances by determining two signalswhich supplement each other in regard to their information content aboutcardiac activity and in that respect however at the same time use onlysuch signals, for the determination of which the computing and memorydemands are particularly low.

In the device according to the invention for detecting tachycardiacdisturbances, based on the features of the classifying portion of claim1, that is attained in that the evaluation unit is adapted to outputboth a first signal which corresponds to the difference of a maximum anda minimum measurement signal within at least one defined period of timeand also a second signal which is dependent on the integral of themeasurement signal over at least one defined period of time.

The defined period of time can extend over a predetermined period oftime, for example 5 seconds, and/or over at least one cardiac cycle. Theduration of a cardiac cycle can be determined for example in anelectrocardiogram (ECG) as the period of time between two identicalcharacteristic points of successive cardiac cycles, for example theQ-points.

The difference in the extreme values of the measurement signal within acardiac cycle, that is to say the peak-to-peak amplitude PPA of themeasurement signal indicates the difference between the intracardialimpedance of the systolic and diastolic heart. During the systole thereis a minimum amount of blood in the heart whereby the measured impedanceassumes a maximum value within the cardiac period. During the diastolethe heart contains a maximum amount of blood which causes the measuredimpedance to assume a minimum. The PPA is therefore a measurement inrespect of the pump activity of the heart. When a tachycardia occurs thePPA of the intracardial impedance is significantly reduced by virtue ofthe reduced pump efficiency of the heart. In the case of a fibrillatingheart the PPA of the intracardial impedance falls to approximately zerobecause the volume of blood in the heart is scarcely variable because ofthe uncoordinated movement of the heart muscles. In that respect thefall in the PPA below a predeterminable (patient-dependent) thresholdvalue is a clear indication of the existence of a tachycardiac rhythmdisturbance.

The PPA can be determined in a manner known to the man skilled in theart in a simple fashion and without complicated computations and withoutthe storage of relatively large amounts of data.

Information about the pump activity of the heart is also collected byvirtue of additionally determining, in accordance with the invention, asecond signal which is dependent on the integral of the measurementsignal over at least one defined period of time. When a tachycardia ordefibrillation exists that second signal, by virtue of the worsened pumpactivity of the heart, assumes different values from the case involvinga heart which is beating normally.

Integration of the measurement signal can also be implemented in per seknown manner in a simple fashion and without involving memorycomplication and expenditure by virtue of the evaluation unit of thedevice according to the invention.

In addition it is advantageous that short-term deflections of themeasurement signal have a substantially lesser effect on the secondsignal than on the PPA. Such deflections occur for example upon avariation in the position of measuring electrodes relative to the vesselwalls of the heart. They can falsify the PPA by virtue of the fact thatsuch a random, unusually high or low measurement signal forms themaximum or minimum during the measurement time interval, under somecircumstances even a plurality of measurement time intervals insuccession. Thus the first signal can incorrectly indicate the existenceof a cardiac arrhythmia which does not exist. However, because of therelatively long period of time over which integration is effected such adeflection has an only slight influence on the integral of themeasurement signal. In that case therefore the second signal willcorrectly indicate normal cardiac activity. That avoids unnecessaryadministration of a cardioversion or defibrillation therapy.

In that respect the items of information which the first and secondsignals furnish about cardiac activity supplement each other and providefor a high level of reliability in the detection of cardiac rhythmdisturbances by the device according to the invention.

The first and the second signal can be determined by evaluation of themeasurement signal within one or more periods of time. For example it ispossible to briefly interrupt and then continue evaluation of themeasurement signal. It is also possible to average the first or secondsignal which is determined over a plurality of periods of time.

The first and second signals however do not always have to be determinedat the same time. For example it can also be provided that it is onlywhen the first signal provides an indication of the existence of anarrhythmia that the additional determination of the second signal isimplemented in a subsequent measurement step.

The duration of one or more cardiac cycles is desirable as theevaluation period of time. In that case, the operation of determiningthe start and stop times of the evaluation procedure relative to thecardiac cycle can be effected with per se known means. It is importantthat the device can alternatively evaluate over predetermined periods oftime which are independent of the cardiac cycle as for example in afibrillation situation triggering relative to the cardiac cycle cannotfunction.

The measures in accordance with the invention ensure on the one handthat electrostimulation therapy is also administered when there is anacute requirement. At the same time the invention avoids therapy whichis unpleasant for the patient being unnecessarily administered solely byvirtue of fluctuations in the measurement signal. The evaluation of twomeasurement signals which supplement each other in terms of theirsignificance therefore affords a substantial increase in the level ofreliability in the detection of tachycardiac rhythm disturbances. On theother hand both measurement signals can be easily determined; theevaluation thereof does not require any major computing expenditure.Simple comparison with reference values or reference value rangesalready indicates the current condition of the heart.

In a preferred embodiment of the invention impedance measurement can becarried out in a unipolar mode. In unipolar impedance measurement,besides the measuring electrode which is introduced for example into theright ventricle, the implanted housing of the device is used at the sametime as the second electrode. By virtue of the relatively large spacingbetween those electrodes, the unipolar impedance signal involves notjust the information about the volume of blood in the heart. Rather forexample information about the respiration rate is also included in theunipolar impedance signal. That can be used separately for ascertainingphysiological parameters for example in the context of a pacemakertherapy. If it is only information about the pump activity of the heartthat is to be processed, the low measurement signal frequenciesoccurring due to respiration (max. 1 Hz) can be easily separated fromthe higher-frequency component of the cardiac activity by suitablefiltering of the signal.

Alternatively impedance measurement in the device according to theinvention can also be implemented in a bipolar mode. In this case ameasuring probe which is introduced into the heart will have twoelectrically mutually insulated electrodes. The flow of current inducedin the measurement procedure is effected through the blood in theventricle. The housing of the device does not play any part here.

In both cases the measuring unit has at least two electrodes of which atleast one can be introduced into a chamber of the heart. In the case ofunipolar measurement the second electrode is formed by an implantablehousing of the device. In the case of bipolar measurement bothelectrodes are arranged directly in the region of the heart. In thatcase the second electrode can be arranged outside the heart, for exampleat the outside of the heart, or it can also be introduced into a chamberof the heart, but not necessarily into the same chamber as the firstelectrode. The essential point is that the flow of current in impedancemeasurement goes through a volume of blood within one or more chambersof the heart, that volume varying in the course of the cardiac period.

Preferably intracardial impedance can be determined in unipolarimpedance measurement and also in bipolar impedance measurement bymeasurement of an electrical voltage between the electrodes when theyare subjected to a preadjustable electrical current. Therefore theoperation of determining impedance is effected on the basis of thepreset current value and the measured voltage value in accordance withOhm's law. The measuring unit accordingly has a current source. It isconnected to the electrodes of the measuring unit in such a way that itproduces a predetermined measuring current between them. The measuringcurrent is, preferably of a pulsed nature, as will be described ingreater detail hereinafter. In this embodiment the device also hasvoltage measuring means which are connected to the electrodes to measurean electrical voltage between the electrodes. In this embodimentimpedance can be determined by division of the measurement signal by thecurrent strength.

The amount of energy E(T) applied for impedance measurement during apredetermined period of time T, with a constant current I, can becalculated easily from the time integral, which is determined in anycase, of the voltage U: E(T) = I × ∫_(T)U(t)  𝕕t

The energy determined in that way can be evaluated as an additionalinformation source.

Alternatively, in the device according to the invention, intracardialimpedance can be determined by measurement of a flow of current betweenthe electrodes when subjected to a preadjustable electrical voltage. Forthat purpose the measuring unit has a voltage source which is connectedto the electrodes of the measuring unit in such a way that it produces apredetermined measuring voltage between them, and current measuringmeans which are connected to the electrodes to measure an electricalcurrent between the electrodes.

In an embodiment of the invention, to avoid polarization of theelectrode, the applied constant current strength or voltage can bemodulated with a predeterminable time dependency in such a way that aperiod alternating current flows or a periodic ac voltage is applied,the maximum current strength of which or the maximum voltage amplitudeof which is of the same value in each half-period. The pulse shape andfrequency can be predetermined. Preferably an alternating currentinvolving a square-wave pulse shape is used. In an embodiment, bipolarsquare-wave pulses with a time spacing of about 50 milliseconds areused. The positive and the immediately adjoining negative half-periodsof the square-wave pulses each involve a duration of about 1microsecond.

By virtue of the differing frequency dependency of the impedance of theblood and the myocardium tissue, the impedance contrast between systoleand diastole can also be increased by a suitable choice of frequency inorder to make evaluation of the PPA still more reliable, in terms ofdetecting cardiac arrhythmia. At a frequency of 4096 Hz approximatelythe specific resistance of blood is only a third of the specificresistance of the myocardium tissue.

In this embodiment the modulation of the measurement signal, which iscaused by the frequency impressed on the measurement current or themeasurement voltage respectively is preferably removed by a demodulationstage connected upstream of the time integration step. In operation ofthe device applied to the input of the demodulation stage is themeasurement signal of the voltage measuring means (or current measuringmeans), which is modulated by the alternating current (or ac voltage)produced by the current source (or voltage source). The demodulationstage is designed in such a way that it is possible to take off at theoutput thereof a signal which corresponds in its configuration inrespect of time to the envelope of the measurement signal or thepositive or negative half-period of the measurement signal.

A further embodiment of the invention has control means which areadapted to cause the evaluation unit to start or stop integration of asignal at the input of the evaluation unit. The control means arepreferably so designed that they cause the evaluation unit to effectintegration over the respective period of time of one or some cardiacperiods. For that purpose it is possible to have recourse to knowntrigger methods. In the case of a normally beating heart integration isstarted at a given phase point in the cardiac period so that normal,non-pathological frequency changes are not crucial in determining theintegral of the measurement signal. The integration duration, that is tosay the predetermined period of time T, is therefore adapted variablywithin certain limits to the cardiac frequency.

The control means however is preferably additionally so designed that itcan cause the evaluation unit to effect integration over at least onerespective predeterminable period of time which is independent of theduration of the cardiac period. That is required in the case of afibrillating heart as triggering here is not possible. If triggeringfails therefore, that is to be assessed as a first indication ofarrhythmia. The device reacts to that situation by switching over to apredetermined fixed period of time for the integration operation. A newimpedance measurement procedure is then started in order to check thecondition of the heart.

In a further embodiment of the invention the evaluation unit has amemory. The evaluation unit is further so designed that, within thepredetermined period of time, the hitherto maximum and minimum signalsapplied to the input of the evaluation unit are continuously determinedand stored in the memory, that at the end of the period of time thedifference between the currently stored maximum and minimum signals iscalculated and that at the beginning of a respective subsequent periodof time the signals last stored in the preceding period of time areerased from the memory. The memory therefore serves here only fortemporarily receiving the values which are currently determined as themaximum and the minimum. They are overwritten as soon as a fresh maximumor minimum of the measurement signal is established within themeasurement time interval. The operation of determining the extremevalues can be re-started with each measurement period.

The period of time for which the operation of determining the PPA isexecuted is preferably also established by control means which areconnected to the evaluation unit and which are so designed that theysignal the evaluation unit the beginning and the end of thepredetermined period of time.

In a particularly advantageous embodiment time control in respect ofdetermining the first and second signals is effected centrally. Thecontrol means are correspondingly so designed that they cause theevaluation unit to effect integration and to determine the differencebetween the maximum and the minimum in each case within the same periodof time. In that way, with the first and second signals, there are twopartially complementary items of information about cardiac activitywithin the same period of time. Arrhythmia can be reliably and quicklydiagnosed.

In a preferred embodiment the evaluation unit is so designed that, inoperation of the device, at the end of the respective period of time, itoutputs such a second signal which corresponds to the integral of themeasurement signal over the period of time less the product of theduration of the period of time and the measurement signal minimum of theperiod of time.

In that way the significance of the second signal can be increased. Thatwill be immediately apparent if it is considered that intracardialimpedance of a fibrillating heart is approximately constant, because ofthe volume of blood which is little variable, and it assumes a valuewhich is lower than the systolic impedance value but markedly higherthan the diastolic impedance value of the heart. The time integral ZI ofthe intracardial impedance of the fibrillating heart will thereforediffer only slightly from that of the normally beating heart over thesame period of time T.

In order to increase significance therefore integration is effected overthe difference between the current impedance value and a minimumimpedance value Z₀(T) which is individual for each integration period T.That value Z₀(T) is in any case constantly fixed in the context ofdetermining the PPA. That minimum impedance value Z₀ corresponds in thenormal situation to the diastolic impedance signal. If therefore theintracardial impedance is denoted by Z, the integration interval by Tand the second signal by ZI(T), then ZI(T) is given by:ZI(T) = ∫_(T)Z(t)  𝕕t − Z₀(T) × T

For the sake of simplicity, without any loss in terms of mathematicalaccuracy, the differencing operation is effected downstream ofintegration and uses the minimum extreme value Z₀(T) of intracardialimpedance Z, which is fixed in the integration period T for determiningthe PPA.

In the case of a fibrillating heart the difference between Z_(o)(T) andZ(t) is so slight that ZI(T) assumes a markedly lower value than in thecase of a normally beating heart. In the condition of tachycardia it isadmittedly possible to detect greater modulation of intracardialimpedance than in the case of fibrillation, but the value of ZI(T) isstill always markedly lower than in the case of a normally beatingheart.

A further embodiment of the invention, for evaluation of the PPA and ZIsignals, has a respective comparator which is connected to a firstmemory for reference values and/or reference value ranges and which isso designed that it compares the difference between successive extremevalues or the time integral of the measurement signal to a respectivereference value or reference value range contained in the first memoryand produces an output signal which indicates whether the respectivecomparison result corresponds to a deviation from the reference value orreference value range or not, and possibly what deviation is involved.The two output signals of the comparators therefore indicate to adownstream-connected processing unit whether the respective signal is inthe standard range or whether it deviates from the standard range. Inthe event of a deviation the amount by which and the direction in whichthe measurement signal deviates from the respective standard range canalso be found from those output signals.

The reference values or reference value ranges which are stored in thefirst memory can be adaptive, that is to say variable, for example inaccordance with the current physical activity, for further enhancing thereliability of detection of tachycardiac rhythm disturbances. For thatpurpose the memory content is influenced by a processing unit whichanalyzes the impedance signal by means of an algorithm, for example a“Regional Effective Slope Quality” (RQ-) algorithm (see Max Schaldach,Electrotherapy of the Heart, Springer Verdag, Berlin, Heidelberg, NewYork, 1992, pages 114 ff). For that purpose however the processing unitcan also be adapted for processing further or other known signals whichreflect physical activity.

The above-described advantages and features of the detection deviceaccording to the invention are of great benefit in an implantableelectrostimulation device for the treatment of tachycardiac rhythmdisturbances.

Particularly desirable is the integration of pacemaker functions intothe therapy unit of the electrostimulation device, in addition tocardioversion/defibrillation. For that purpose the therapy unit of theelectrostimulation device is so designed that it can optionally alsoapply a pacemaker electrostimulation therapy to the heart. The pacemakertherapy can be applied by way of the same electrode with which impedanceis also measured. In that case no impedance measurement is effectedduring the duration of the stimulation pulse. The control unitimplements the time control of the measuring unit and the therapy unit,which is required for that purpose.

A preferred embodiment of the electrostimulation device according to theinvention additionally has a signal pattern memory which is connected tothe control unit and in which one or more control signals for themeasuring unit, the evaluation unit and/or the therapy unit areassociated with signal patterns, that is to say output signals orcombinations of output signals from the evaluation unit. The controlunit is so designed that it compares output signals received from theevaluation unit to the signal patterns of the signal pattern memory andproduces control signals associated with the respectively correct signalpattern and transmits them to the corresponding unit.

For example the control unit receives output signals from the evaluationunit, which indicate that the PPA value is 10% below the associatedreference value range while the ZI value does not deviate from thereference value. The control unit compares that signal pattern to thosestored in the signal pattern memory and recognizes from the entry whichis correct in that case that a first control signal is to be sent to themeasuring unit for again executing the operation of determining the PPAand ZI values, and a second control signal is to be sent to the therapyunit for continuing normal pacemaker therapy with the parameters appliedhitherto. The control unit executes those steps.

Alternatively or supplemental to control of the electrostimulationdevice on the basis of the output signals of the evaluation unit bymeans of the signal pattern memory, the control unit can also access oneor more assessment algorithms contained in a program memory in order toassess the output signals of the evaluation unit and to determine andproduce the necessary control signals. A computing unit is provided forexecuting the assessment algorithm.

BRIEF DESCRIPTION OF THE FIGURE

Further advantages of the invention will be apparent from thedescription hereinafter of an embodiment with reference to the drawing.

FIG. 1 shows a block diagram of an electrostimulation device accordingto the invention for treating tachycardiac rhythm disturbances withintegrated pacemaker functions.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The stimulation device 10 shown in FIG. 1 involves an implantableintegrated pacemaker and cardioverter/defibrillator which is used forthe stimulation of a human heart 12.

For impedance measurement a measuring electrode 14 is introduced throughthe right atrium 16 into the right ventricle 18 of the heart 12. In theregion of its distal end the measuring electrode 14 has an electrode 20at which, in the impedance measurement procedure, an electricalmeasuring current issues into the right ventricle 18 or the bloodcontained therein.

The measuring current is an electrical alternating current ofsquare-wave pulse shape and with the same value of current strength inboth half-periods. The measuring current is produced within an assembly22 in a measuring unit 23 by a current source 24 using a modulator 26.The function of the current source 24 is controlled by a central control28 and synchronized with other functional procedures. For example themeasuring electrode 20 can be used at the same time for stimulation ofthe right ventricle in the context of pacemaker therapy. Impedancemeasurement is then stopped by the control 28, for the duration of thestimulation pulse.

Impedance measurement is effected in a unipolar mode. Therefore theelectrical voltage between the electrode 20 and the housing 36 of theassembly 22 is measured by way of suitable signal lines 30 and 32 bymeans of a voltmeter 34. The voltage signal is cleaned of the modulationfrequency in a demodulator 38 connected on the downstream side thereof.

A filter stage 40 which can be selectively activated or deactivated bythe control 28 serves to clean the measurement signal of frequencycomponents in the range of up to about 1 Hz (respiration rate). Thefiltered measurement signal is then digitized by an A/D converter 41.The measuring unit 23 therefore outputs a digitized value correspondingto the measured voltage.

The voltage values outputted with the passage of time at the output ofthe measuring unit 23 are processed in an evaluation unit 22 in parallelin an extreme value analyzer 44 and in an integration and differencestage 46. The two evaluation stages 44 and 46 process the voltage valuesat the input of the evaluation unit 42 over equal periods of time. Thelength of that period of time is determined in each case by the control28. It can determine the period of time on the basis of precedingevents, computations or triggering.

The extreme value analyzer 44 compares the voltage value currentlyoccurring at its input to a respective minimum and maximum value whichhave been previously put into intermediate storage. If the currentvoltage value is greater than the maximum value then that voltage valueis written as a new maximum value into the intermediate memory and theold one is erased. If the current voltage value is less than the minimumvalue that minimum value is overwritten by the current voltage value. Ifneither of the two specified relationships is satisfied the procedureinvolves waiting for the next voltage value. In response to a signalfrom the control unit 28 the difference is formed between the maximumand the minimum values and outputted to a first comparator 48. Theintermediate memories are generally erased for the beginning ofevaluation of the measurement signal of a fresh measurement time periodT, unless the control 28 caused only a short interruption inmeasurement.

In the integration and differentiation stage 46 the voltage values whichsuccessively occur at the input are integrated over an integrationperiod which is monitored and established by the control 28. Integrationis concluded in response to a corresponding signal from the control 28.The control 28 causes the extreme value analyzer 44 to output the setminimum value from the intermediate memory. Here too it is provided thatintegration is only to be suspended in response to a correspondingsignal from the control 28 and continued in response to a furthercontrol signal without output being effected in the interim. In that wayintegration can be effected over a plurality of cardiac periods withoutrecording deflections of the measurement signal which are caused by anelectrical stimulation pulse.

The output signals from the extreme value analyzer 44 and theintegration/difference stage 46 are compared in respective comparators48 and 50 to reference values (or reference value ranges). For thatpurpose the comparators access a reference value memory 52 and form thedifference between the output signal and the respective reference value(or the boundary values of the reference value range). The differencevalues formed in that way are transmitted to the control 28 by theevaluation unit 42 as results.

For assessment of the evaluation results the control 28 accesses asignal pattern memory 54. It contains signal patterns, that is to sayoutput signals or combinations of output signals from the evaluationunit. Associated with each signal pattern in the signal pattern memoryis one or more control signals serving for control of the measuringunit, the evaluation unit and/or the therapy unit, in a mannerappropriate to the situation. The therapy unit Includes a pacemaker unit56 and a cardioversion/defibrillation unit 58.

The output signals supplied by the evaluation unit are compared to thesignal patterns of the signal pattern memory. The control ascertainsfrom the signal pattern memory 54 which control signals are to beproduced for which unit in relation to the respectively establishedsignal pattern. In that way, depending on the respective situationestablished, measurement is repeated or a therapy is initiated orcontinued. A further reaction to the output signals of the evaluationunit 42 can also be adaptation of the reference value ranges, forexample in the case of increased physical activity. For that purpose thenew reference values (or reference value ranges) are taken from thesignal pattern memory and written into the reference value memory 52.

Alternatively or in addition it is also possible to access an assessmentalgorithm which is stored in a program memory (not shown), for theexecution of which by means of a computing unit (also not shown) theoutput signals of the evaluation unit are inputted as parameters. Asabove the control 28 obtains the control signals to be produced, as theresult of execution of such an assessment algorithm.

1. A device for detecting tachycardiac rhythm disturbances, comprising:a measuring unit for picking up and reproducing a measurement signaldependent on intracardial impedance at an output thereof; and anevaluation unit for detecting tachycardiac rhythm disturbances which isconnected at an input side thereof to the measuring unit and which isadapted to determine an output a first signal which corresponds to thedifference of a maximum and a minimum measurement signal within at leastone defined period of time, wherein the evaluation unit is additionallyadapted to determine and output a second signal which is dependent onthe integral of the measurement signal over at least one defined secondperiod of time, and the first signal associated with the second periodof time, wherein the first-mentioned period of time is before the secondperiod of time or is identical to the second period of time.
 2. Thedevice of claim 1, further comprising: means for measuring unipolarimpedance.
 3. The device of claim 1, further comprising: means formeasuring bipolar impedance.
 4. The device of claim 1, wherein themeasuring unit comprises at least two electrodes, of which at least onecan be introduced into a chamber of the heart.
 5. The device of claim 4,further comprising: a current source connected to the electrodes in sucha way that it produces a predetermined measuring current between theelectrodes; and voltage measuring means connected to the electrodes formeasuring an electrical voltage therebetween.
 6. The device of claim 4,further comprising: a voltage source connected to the electrodes in sucha way that it produces a predetermined measuring voltage between theelectrodes, and current measuring means connected to the electrodes formeasuring an electrical current therebetween.
 7. The device of claim 5,wherein the current source is adapted to output an alternating currentwith a predetermined time dependency.
 8. The device of claim 7, furthercomprising: a demodulation device, at an input of which in operation ofthe device is the measurement signal of the voltage measuring means,which is modulated by the alternating current produced by the currentsource, the demodulation device being so designed that a signal can betaken off at an output, which corresponds in its configuration inrespect of time to the envelope of the measurement signal or thepositive or negative half-period of the measurement signal.
 9. Thedevice of claim 1, further comprising: a filter stage for the removal ofmeasurement signal components of a frequency of up to a maximum of 1 Hz.10. The device of claim 1, further comprising: control means adapted tocause the evaluation unit to start or stop integration of a signal atthe input of the evaluation unit.
 11. The device of claim 10, whereinthe control means is so designed to cause the evaluation unit to effectintegration respectively over the period of time of one or some cardiacperiods.
 12. The device of claim 10, wherein the control means isdesigned to cause the evaluation unit to effect integration over atleast one respective predeterminable period of time which is independentof the cardiac period duration.
 13. The device of claim 10, wherein theevaluation unit has a memory.
 14. The device of claim 13, wherein theevaluation unit is so designed that within the predetermined period oftime the hitherto maximum and minimum signals at the input of theevaluation unit are continuously determined and stored in the memory,that at the end of the period of time the difference between thecurrently stored maximum and minimum signals is calculated and that atthe beginning of a respective subsequent period of time the signals laststored in the preceding period of time are erased from the memory. 15.The device of claim 14, wherein the control means is connected to theevaluation unit and designed to signal to the evaluation unit thebeginning and the end of the predetermined period of time.
 16. Thedevice of claim 15, wherein the control means is designed to cause theevaluation unit to effect integration and to determine the differencebetween the maximum and the minimum respectively within the same periodof time.
 17. The device of claim 16, wherein the evaluation unit is sodesigned that in operation of the device at the end of the respectiveperiod of time the evaluation unit outputs a signal which corresponds tothe integral of the measurement signal over the period of time less theproduct of the duration of the period of time and the measurement signalminimum of the period of time.
 18. The device of claim 17, furthercomprising: a respective comparator connected to a reference valuememory containing reference values and/or reference value ranges andwhich is so designed that in operation of the device the comparatorcompares the difference of the extreme values or the time integral ofthe measurement signal to a respective reference value or referencevalue range contained in the reference value memory and produces anoutput signal which indicates whether the respective comparison resultcorresponds to a deviation from the reference value or reference valuerange or not and possibly what deviation is involved.
 19. The device ofclaim 17, further comprising: a respective comparator connected to areference value memory containing fluctuation reference values and/orfluctuation reference value ranges and which is so designed that inoperation of the device the comparator compares the change in thedifference of the extreme values or the time integral of the measurementsignal in relation to the respectively precedingly determined value to arespective fluctuation reference value or fluctuation reference valuerange contained in the reference value memory and produces an outputsignal which indicates whether the respective comparison resultcorresponds to a deviation from the fluctuation reference value orfluctuation reference value range or not and possibly what deviation isinvolved.
 20. An implantable electrostimulation device for the treatmentof tachycardiac rhythm disturbances comprising: a detection unit; acontrol unit; a therapy unit for the treatment of tachycardiac rhythmdisturbances adapted to produce a cardioversion or defibrillationelectrotherapy to be applied to the heart, wherein the detection unitcomprises: a measuring unit for picking up and reproducing a measurementsignal corresponding to an intracardial impedance at an output thereof;and an evaluation unit connected at an input to the measuring unit, andwherein the control unit receives output signals from the evaluationunit and controls the activity of the measuring unit, the evaluationunit and the therapy unit, and wherein the evaluation unit is adapted todetermine and output a first signal which corresponds to the differenceof a maximum and a minimum measured signal within at least one definedperiod of time, and wherein the evaluation unit is additionally adaptedto determine and output a second signal which is dependent on theintegral of the measurement signal over at least one defined secondperiod of time, and the first signal associated with the second periodof time, wherein the first-mentioned period of time is before the secondperiod of time or is identical to the second period of time.
 21. Theelectrostimulation device of claim 20, wherein the therapy unit is sodesigned that it can also selectively apply a pacemakerelectrostimulation therapy to the heart.
 22. The electrostimulationdevice of claim 21, further comprising: a signal pattern memoryconnected to the control unit and in which one or more control signalsfor the measuring unit, the evaluation unit and/or the therapy unit areassociated with signal patterns, that is to say, output signals orcombinations of output signals from the evaluation unit, and that thecontrol unit is so designed that it compares output signals receivedfrom the evaluation unit to the signal patterns of the signal patternmemory and produces the control signals associated with the respectivelycorrect signal pattern and transmits same to the corresponding unit. 23.The electrostimulation device of claim 22, wherein, for determining thecontrol signals, the control unit additionally or exclusively accessesan assessment algorithm which is contained in a program memory and bymeans of which the control signals to be produced are computed in acomputing unit on the basis of the output signals of the evaluationunit.
 24. The electrostimulation device of claim 23, further comprising:a housing which can be used as an electrode in the measurement ofintracardial impedance.
 25. The device of claim 2, wherein the measuringunit comprises at least two electrodes, of which at least one can beintroduced into a chamber of the heart.
 26. The device of claim 3,wherein the measuring unit comprises at least two electrodes, of whichat least one can be introduced into a chamber of the heart.
 27. Thedevice of claim 25, further comprising: a current source connected tothe electrodes in such a way that it produces a predetermined measuringcurrent between the electrodes; and voltage measuring means connected tothe electrodes for measuring an electrical voltage therebetween.
 28. Thedevice of claim 26, further comprising: a current source connected tothe electrodes in such a way that it produces a predetermined measuringcurrent between the electrodes; and voltage measuring means connected tothe electrodes for measuring an electrical voltage therebetween.
 29. Thedevice of claim 25, further comprising: a voltage source connected tothe electrodes in such a way that it produces a predetermined measuringvoltage between the electrodes, and current measuring means connected tothe electrodes for measuring an electrical current therebetween.
 30. Thedevice of claim 26, further comprising: a voltage source connected tothe electrodes in such a way that it produces a predetermined measuringvoltage between the electrodes, and current measuring means connected tothe electrodes for measuring an electrical current therebetween.
 31. Thedevice of claim 6, wherein the voltage source is adapted to output an acvoltage with a predetermined time dependency.
 32. The device of claim27, wherein the current source is adapted to output an alternatingcurrent with a predetermined time dependency.
 33. The device of claim28, wherein the current source is adapted to output an alternatingcurrent with a predetermined time dependency.
 34. The device of claim29, wherein the voltage source is adapted to output an ac voltage with apredetermined time dependency.
 35. The device of claim 30, wherein thevoltage source is adapted to output an ac voltage with a predeterminedtime dependency.
 36. The device of claim 31, further comprising: ademodulation device, at an input of which in operation of the device isthe measurement signal of the current measuring means, which ismodulated by the ac voltage produced by the voltage source, thedemodulation device being so designed that a signal can be taken off atan output, which corresponds in its configuration in respect of time tothe envelope of the measurement signal or the positive or negativehalf-period of the measurement signal.
 37. The device of claim 32,further comprising: a demodulation device, at an input of which inoperation of the device is the measurement signal of the voltagemeasuring means, which is modulated by the alternating current producedby the current source, the demodulation device being so designed that asignal can be taken off at an output, which corresponds in itsconfiguration in respect of time to the envelope of the measurementsignal or the positive or negative half-period of the measurementsignal.
 38. The device of claim 33, further comprising: a demodulationdevice, at an input of which in operation of the device is themeasurement signal of the voltage measuring means, which is modulated bythe alternating current produced by the current source, the demodulationdevice being so designed that a signal can be taken off at an output,which corresponds in its configuration in respect of time to theenvelope of the measurement signal or the positive or negativehalf-period of the measurement signal.
 39. The device of claim 34,further comprising: a demodulation device, at an input of which inoperation of the device is the measurement signal of the currentmeasuring means, which is modulated by the ac voltage produced by thevoltage source, the demodulation device being so designed that a signalcan be taken off at an output, which corresponds in its configuration inrespect of time to the envelope of the measurement signal or thepositive or negative half-period of the measurement signal.
 40. Thedevice of claim 35, further comprising: a demodulation device, at aninput of which in operation of the device is the measurement signal ofthe current measuring means, which is modulated by the ac voltageproduced by the voltage source, the demodulation device being sodesigned that a signal can be taken off at an output, which correspondsin its configuration in respect of time to the envelope of themeasurement signal or the positive or negative half-period of themeasurement signal.
 41. The device of claim 37, further comprising: afilter stage for the removal of measurement signal components of afrequency of up to a maximum of 1 Hz.
 42. The device of claim 39,further comprising: a filter stage for the removal of measurement signalcomponents of a frequency of up to a maximum of 1 Hz.
 43. The device ofclaim 41, further comprising: control means adapted to cause theevaluation unit to start or stop integration of a signal at the input ofthe evaluation unit.
 44. The device of claim 42, further comprising:control means adapted to cause the evaluation unit to start or stopintegration of a signal at the input of the evaluation unit.
 45. Thedevice of claim 43, wherein the control means is so designed to causethe evaluation unit to effect integration respectively over the periodof time of one or some cardiac periods.
 46. The device of claim 44,wherein the control means is so designed to cause the evaluation unit toeffect integration respectively over the period of time of one or somecardiac periods.
 47. The device of claim 11, wherein the control meansis designed to cause the evaluation unit to effect integration over atleast one respective predeterminable period of time which is independentof the cardiac period duration.
 48. The device of claim 43, wherein thecontrol means is designed to cause the evaluation unit to effectintegration over at least one respective predeterminable period of timewhich is independent of the cardiac period duration.
 49. The device ofclaim 44, wherein the control means is designed to cause the evaluationunit to effect integration over at least one respective predeterminableperiod of time which is independent of the cardiac period duration. 50.The device of claim 45, wherein the control means is designed to causethe evaluation unit to effect integration over at least one respectivepredeterminable period of time which is independent of the cardiacperiod duration.
 51. The device of claim 46, wherein the control meansis designed to cause the evaluation unit to effect integration over atleast one respective predeterminable period of time which is independentof the cardiac period duration.
 52. The device of claim 50, wherein theevaluation unit has a memory.
 53. The device of claim 51, wherein theevaluation unit has a memory.
 54. The device of claim 52, wherein theevaluation unit is so designed that within the predetermined period oftime the hitherto maximum and minimum signals at the input of theevaluation unit are continuously determined and stored in the memory,that at the end of the period of time the difference between thecurrently stored maximum and minimum signals is calculated and that atthe beginning of a respective subsequent period of time the signals laststored in the preceding period of time are erased from the memory. 55.The device of claim 53, wherein the evaluation unit is so designed thatwithin the predetermined period of time the hitherto maximum and minimumsignals at the input of the evaluation unit are continuously determinedand stored in the memory, that at the end of the period of time thedifference between the currently stored maximum and minimum signals iscalculated and that at the beginning of a respective subsequent periodof time the signals last stored in the preceding period of time areerased from the memory.
 56. The device of claim 54, wherein the controlmeans is connected to the evaluation unit and designed to signal to theevaluation unit the beginning and the end of the predetermined period oftime.
 57. The device of claim 55, wherein the control means is connectedto the evaluation unit and designed to signal to the evaluation unit thebeginning and the end of the predetermined period of time.
 58. Thedevice of claim 56, wherein the control means is designed to cause theevaluation unit to effect integration and to determine the differencebetween the maximum and the minimum respectively within the same periodof time.
 59. The device of claim 57, wherein the control means isdesigned to cause the evaluation unit to effect integration and todetermine the difference between the maximum and the minimumrespectively within the same period of time.
 60. The device of claim 58,wherein the evaluation unit is so designed that in operation of thedevice at the end of the respective period of time the evaluation unitoutputs a signal which corresponds to the integral of the measurementsignal over the period of time less the product of the duration of theperiod of time and the measurement signal minimum of the period of time.61. The device of claim 59, wherein the evaluation unit is so designedthat in operation of the device at the end of the respective period oftime the evaluation unit outputs a signal which corresponds to theintegral of the measurement signal over the period of time less theproduct of the duration of the period of time and the measurement signalminimum of the period of time.
 62. The device of claim 60, furthercomprising: a respective comparator connected to a reference valuememory containing reference values and/or reference value ranges andwhich is so designed that in operation of the device the comparatorcompares the difference of the extreme values or the time integral ofthe measurement signal to a respective reference value or referencevalue range contained in the reference value memory and produces anoutput signal which indicates whether the respective comparison resultcorresponds to a deviation from the reference value or reference valuerange or not and possibly what deviation is involved.
 63. The device ofclaim 61, further comprising: a respective comparator connected to areference value memory containing reference values and/or referencevalue ranges and which is so designed that in operation of the devicethe comparator compares the difference of the extreme values or the timeintegral of the measurement signal to a respective reference value orreference value range contained in the reference value memory andproduces an output signal which indicates whether the respectivecomparison result corresponds to a deviation from the reference value orreference value range or not and possibly what deviation is involved.64. The device of claim 60, further comprising: a respective comparatorconnected to a reference value memory containing fluctuation referencevalues and/or fluctuation reference value ranges and which is sodesigned that in operation of the device the comparator compares thechange in the difference of the extreme values or the time integral ofthe measurement signal in relation to the respectively precedinglydetermined value to a respective fluctuation reference value orfluctuation reference value range contained in the reference valuememory and produces an output signal which indicates whether therespective comparison result corresponds to a deviation from thefluctuation reference value or fluctuation reference value range or notand possibly what deviation is involved.
 65. The device of claim 61,further comprising: a respective comparator connected to a referencevalue memory containing fluctuation reference values and/or fluctuationreference value ranges and which is so designed that in operation of thedevice the comparator compares the change in the difference of theextreme values or the time integral of the measurement signal inrelation to the respectively precedingly determined value to arespective fluctuation reference value or fluctuation reference valuerange contained in the reference value memory and produces an outputsignal which indicates whether the respective comparison resultcorresponds to a deviation from the fluctuation reference value orfluctuation reference value range or not and possibly what deviation isinvolved.
 66. The electrostimulation device of claim 20, furthercomprising: a signal pattern memory connected to the control unit and inwhich one or more control signals for the measuring unit, the evaluationunit and/or the therapy unit are associated with signal patterns, thatis to say, output signals or combinations of output signals from theevaluation unit, and that the control unit is so designed that itcompares output signals received from the evaluation unit to the signalpatterns of the signal pattern memory and produces the control signalsassociated with the respectively correct signal pattern and transmitssame to the corresponding unit.
 67. The electrostimulation device ofclaim 22, wherein, for determining the control signals, the control unitadditionally or exclusively accesses an assessment algorithm which iscontained in a program memory and by means of which the control signalsto be produced are computed in a computing unit on the basis of theoutput signals of the evaluation unit.
 68. The electrostimulation deviceof claim 23, further comprising: a housing which can be used as anelectrode in the measurement of intracardial impedance.
 69. Theelectrostimulation device of claim 20, further comprising: a housingwhich can be used as an electrode in the measurement of intracardialimpedance.